1. Field of the Invention
The present invention relates to a control device of a switching power supply, and in particular to improvement of stability of an output voltage in a process of change in a setting value of the output voltage setting in a current resonant converter, a type of switching power supply.
2. Description of the Related Art
Current resonant type converters have been used for high efficient switching power supplies with reduced energy losses. A conventional current resonant converter has a construction described in the following.
FIG. 10 shows an example of construction of a conventional current resonant converter. FIG. 11 shows an input-output characteristic, a switching frequency dependence of an input-output conversion ratio, of a current resonant converter section.
The current resonant converter having the construction of FIG. 10 comprises a current resonant converter section 100 and a control section 110. The current resonant converter section 100 includes two switching elements M1 and M2 connected in series, to which an input voltage Ed is applied. The switching element M1 is connected in parallel with a series circuit of a resonant capacitor Cr, a resonant inductance Lr, and a magnetizing inductance Lm. The magnetizing inductance Lm is connected in parallel with a primary winding Lp of a transformer T. The transformer T has secondary windings Ls1 and Ls2 provided with a center tap. Each end of the secondary winding of the transformer T is connected to an anode of each of two rectifying elements D1 and D2. Cathodes of the rectifying elements D1 and D2 are connected with each other and connected to the positive terminal of output terminals. The negative terminal of the output terminals is connected to the center tap between the secondary windings Ls1 and Ls2. A smoothing capacitor Co is connected between the cathodes of the rectifying elements D1 and D2, and to the center tap of the secondary windings Ls1 and Ls2.
In the current resonant converter section 100, a resonant circuit is constructed in the primary winding side of the transformer T with the two switching elements M1 and M2, the resonant inductance Lr, the resonant capacitor Cr, and the magnetizing inductance Lm. In a certain condition, the primary winding Lp of the transformer becomes a part of the resonance circuit. In the secondary side of the transformer T, a rectifying circuit is constructed with the two rectifying elements D1 and D2, and the smoothing capacitor Co.
The control section 110 comprises an output voltage detector 111 that receives a DC output voltage Vout and a target voltage and delivers an error signal Ve, an isolator 112, a voltage-frequency converter 113 that receives an output signal V1 from the isolator 112 and delivers a pulse waveform signal with a switching frequency fsw, and a driver circuit 114. The driver circuit 114 delivers control signals Vg1 and Vg2 for ON/OFF controlling the switching elements M1 and M2 of the current resonant converter section 100.
In the control section 110, the output voltage detector 111 detects an error between the DC output voltage Vout and the target voltage and delivers an error signal Ve representing the error, which is received by the isolator 112 that delivers an output signal V1 that is electrically isolated from the error signal Ve. The output signal V1 is converted into a switching signal having a desired switching frequency fsw by the voltage-frequency converter 113. The switching signal is converted by the driver circuit 114 into control signals Vg1 and Vg2 to supply to the two switching elements M1 and M2.
Thus, the control section 110 controls the switching elements in the current resonant converter section 100, and the current resonant converter performs as a DC-DC converter that delivers a desired DC output voltage Vout from the input voltage Ed.
In a current resonant converter, a DC output voltage Vout is generally controlled by varying the switching frequencies of the switching elements M1 and M2, as disclosed in Patent Document 1, for example. The two switching elements M1 and M2 are turned ON/OFF complementarily, and preferably switched with the same ON time, which means the duty ratio of 50%, according to Patent Documents 1 and 2 (identified further on).
The current resonant converter of the figure has two resonant frequencies fr1 and fr2 (fr1<fr2) and the switching frequency of the switching elements M1 and M2 varies between the resonant frequency fr1 and the resonant frequency fr2. FIG. 11 shows a relationship between an input-output conversion ratio G and a switching frequency fsw. It is known that the input-output conversion ratio G exhibits a peak at the lower resonant frequency fr1, as disclosed in Patent Document 3 (identified further on). The resonant frequencies fr1 and fr2 are represented by the formula (1) and (2) below.fr1=1/[2π√((Lr+Lm)*Cr)]  (1)fr2=1/[2π√(Lr*Cr)]  (2)As shown in FIG. 11, switching loss sharply increases when the switching frequency decreases below the lower resonant frequency fr1. Thus, the switching frequency is used in the range not lower than the resonant frequency fr1. At the higher resonant frequency fr2, the input-output conversion ratio G equals 1, which means that the primary side voltage directly appears at the output voltage. In switching frequencies above the higher resonant frequency fr2, the input-output conversion ratio G does not change despite change of the switching frequency as shown in FIG. 11. As a result, the output voltage scarcely changes, which means control of the output voltage is virtually impossible.
For this reason, the switching frequency is varied in the range from the resonance frequency fr1 to the resonance frequency fr2 to control the output voltage. The output voltage is increased by decreasing the switching frequency in the range between the resonance frequencies fr1 and fr2, and the output voltage is decreased by increasing the switching frequency.
In the construction of FIG. 10, the output voltage detector 111 detects the error between the DC output voltage Vout and the target voltage, and the voltage-frequency convertor 113 generates a pulse waveform signal having a switching frequency fsw corresponding to the error voltage. The driver circuit 114, receiving the pulse waveform signal having a switching frequency fsw, gives the control signals Vg1 and Vg2 to the two switching elements M1 and M2. Thus, the current resonant converter section 100 is controlled to be settled to a desired output voltage.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2006-109566
[Patent Document 2]
Japanese Translation of PCT International Application: 2003-510001 corresponding to International Publication Number WO01/20758
[Patent Document 3]
Japanese Unexamined Patent Application Publication No. 2012-249363
However, the input-output characteristic of FIG. 11 shows a nonlinear characteristic. The variation of the input-output conversion ratio G, or the variation of the DC output voltage Vout with respect to the variation of a switching frequency, differs depending on the output voltage setting value or the target voltage. As a result, when a setting value of output voltage is changed, the output voltage varies slowly to the new output voltage value in the range of low output voltages, but in the range of high output voltages, the output voltage varies abruptly causing overshooting, which may subject the load to an overvoltage and sometimes destroy the load. If the overshoot occurs under a light load condition, in particular, it takes a long settling time to recover the target voltage.
In order to avoid this problem, it could be considered that the control gain was decreased in the control section including the output voltage detector and the voltage-frequency convertor. However, in cases of low output voltages, this measure elongates greatly the time period for the output voltage to settle to the target voltage after a change of the setting value of the output voltage. Therefore, the means of decreasing the control gain of the control section cannot be employed for avoiding the overshoot. Although the above Patent Documents describe control of current resonant converters, they do not mention a response to change of a setting value of output voltage.